Process of manufacturing dry rectifiers



Patented Dec. 13, 1938 PATENT OFFICE PROCESS or MANUFACTURING par RECTI- FIERS Etienne Barrey, Nanterre, France No Drawing. Application January 12, 1937, Serial No. 120,181. In France January 16, 1936 12' Claims.

The present invention relates to a process of manufacturing dry rectifiers particularly applicable to the manufacture of elements consisting of a part made of copper, generally having the shape of a disc or a plate, covered with a layer of copper oxide, the copper oxide-copper contact 1 having a dissymmetrical conductibility used for rectifying alternating currents.

It is known that the rectifying elements of this 1 kind are generally manufactured by treating the copper discs or plates in an oven, at a high temperature of about 1000 C. where these parts are superficially oxidized. When oxidized said parts are either rapidly cooled in air or in water or 15 subjected before they are taken out of the oven to a retarded cooling by being left for a period in a second oven heated to an intermediate temperature between that of the oxidation oven and that of the outer air, the layer of oxide crystallizing during the cooling process.

It has however been noticed that this method of treatment, in addition to the dificulties to which it gives rise, does not in practice enable identical results to be obtained for all the parts simultaneously treated and this lack of uniformity is the cause of a considerable amount of scrap.

On the other hand, during the treatment in the oven, the copper parts become covered in the first place with a layer of red cuprous oxide, and

on the other hand with a superficial layer of black cupric oxide covering the first layer. Whereas the red cuprous oxide has a low ohmic resistance,

.the black cupric oxide offers a high resistance to.

the passage of the current, thereby making it necessary to remove said layer of black oxide by a mechanical or chemical treatment, in particular by means of a caustic or acid solution.

Finally, to obtain a good contact of the outer circuit with the oxidized face of the rectifying 4,0 element, it is necessary to cover said face with coke powder or with a metallic deposit obtained by painting, spraying or by electro-plating, the

metallic deposits effected by the known processes being moreover generally unstable and ad- 45 hering badly to the surface of the copper oxide.

It has also been proposed to form a good conducting contact surface by reducing the layer of black oxide by cooling the part in a reducing liquid,

but it is very difiicult to prevent the reduction 50 from taking place on the edges of the disc and from attacking thereon also the red oxide, subsequently causing short circuits.

The invention has in the first place for its object a process enabling perfectly uniform recti- 55 fying elements to be obtained which are identical with each other; said process is characterized by the fact that it consists intreating the previously oxidized copper elements, during the crystallization phase of the oxide layer, by passing an electric current through said layer, this treatment 5 taking place in a good conducting and non-oxidizing medium such as graphite, manganese dioxide, powdered copper oxide or the like.

The action thus exerted by the electric current on the copper oxide crystals in the course of 10 formation is of a purely physical nature and has the effect of uniformly orienting all such crystals, thereby producing elements which on the one handare very uniform and which ofier on the other hand a lower resistance to the passage of 1B the current in the red oxide-copper direction, and a higher resistance in the opposite direction.

The. invention has furthermore for its object to enable the formation of a good surface of contact to be obtained on an oxidized surface, by 20. reducing the black oxide and by an electrolytic deposit, as will be mentioned. hereinafter in a more detailed manner.

By way of example, one mode of carrying out the'process which is the object of the invention 25 has been described hereinafter, applied to copper rectifying elements oxidized in an oven at high temperature, it being however understood that the treatment according to the invention applies to'any body oroxide in the course of crystalliza- 30 tion, whatever be the process used for producing the oxidation proper.

As mentioned above, the copper discs or plates are first of all treated in a known mannerin an oven at about 1000 C., and as soon as the oxida- 35 tion attains the desired degree, the parts are abruptly dipped into a good conducting and nonoxidizing medium which can consist of graphite in powder or in compressed cakes, of a fused mineral salt, of manganese dioxide, of powdered 40 copper oxide or the like, this conducting bath having been previously raised'to a temperature approximating that of the oxidation oven. An electric tension is then applied between an electrode made of any metal and the parts to be treated, in such a manner that the current passes through the layers of oxide in the course of crystallization while the conducting bath slowly cools. The intensity of the current used for this treatment is fairly high and can attain several amperes per cm. of the surface treated.

In the case in which the oxidationof the parts takes place at a low or medium temperature, in an oxidizing or also conducting medium, a suitable electric tension applied in such a manner that it afiects the crystalline formation of the oxides produced, causes the same phenomena to take place as those described with reference to the treatment applied to the oxides obtained at high temperature.

Apart from the above mentioned improvements in the quality of the product thus obtained, it

'has furthermore been noticed that the electrical treatment which is the object of the invention has the effect of decreasing the ohmic resistance of the black cupric oxide superposed on the red cuprous oxide, so that the removal of this layer of black oxide, which is necessary for the elements obtained by the known heat treatment, is no longer necessary.

Nevertheless, to ,produce a good contact between the surface of the red copper oxide and the outer circuit, it is useful to be able to correctly reduce the layer of black oxide and optionally a fraction of the thickness of the underlying layer of red oxide, so as to create a good conducting metal surface in intimate contact with the said red oxide. The present invention enables the reduction to be effected by electrochemical means overan accurately limited zone of the surface of the treated part, without any risk of damaging the edges of the part and therefore without subsequently causing reverse currents which annihilate the rectifying effect.

For this purpose, the oxidized parts are treat-.

ed according to the invention, in an electrolytic bath comprising a conducting salt dissolved in water, the current being passed between the treated part and an electrode made of a reducing metal such as iron, brought into the immediate vicinity of or preferably into direct contact with the surface to be reduced, so that reduction takes place uniformly in the zone which is in contact with said electrode. The electrolyte will advantageously be a reducer, for example a sulfite, bisulflte or bydrosulflte of sodium, an oxide of iron dissolved in water or similar salt. It should be noted that in this treatment which takes place at room temperature, the reduction of the black oxide is extremely rapid, whereas the ensuing reduction of the red oxide is very slow, so that it is an easy matter to control the duration of the operation and to stop it before the part has been damaged by carrying the reduction of the layer of red oxide too far.

Experience has shown'that the black oxide left on the surface of the copper disc in no way impedes the electro-chemical reduction operations. It even appears that the presence of the black oxide facilitates obtaining parts which better withstand electrical over-voltages and permits of lowering the value of the resistance offered to the passage of the current in the oxides in the direction of least resistance.

According to another mode of carrying out the invention, a layer of reduced copper can be obtained at the same time as the electrical treatment to which thepre-oxidized parts are subjected; for this purpose the conducting medium which is brought to a high temperature consists of a reducing body such as graphite in fine grains from which. all silicious gangue has been removed.

The thickness of the layer of reduced copper thus formed depends on the degree of activity of said conducting medium, on its temperature and on the intensity of the current used, which while exerting the above mentioned enhancing influence on the crystalline system in formation can cause an elevation of temperature enhancing the reducing phenomenon.

In view of the relatively small thickness of the layer of black copper oxide covering the blanks, the layer'of copper obtained by reduction is frequently too small for the requirements of industrial practice. Consequently this has led to increasing the thickness of said layer of reduced copper by means of an electrolytic deposit. However, if for this purpose the known process is applied which consists in passing a direct. current between the copper blank and an electrode which is either in contact or not with said blank, said electrode and said blank being immersed in a solution of a salt of the metal which is to be deposited, a deposit is obtained which adheres very badly to the already existing layer of reduced copper and furthermore, the acid ions liberated in the course of the decomposition of the metal salt by the electric current, attack the copper oxide and impair the rectifying properties of the element.

The inventor has discovered that on the contrary by applying an alternating difierence of potential between two electrodes made of different metals, for example made of silver and of zinc, placed in contact with the layer of reduced copper on the same face of the treated disc immersed in a solution of a copper salt, with the optional addition of a reducing salt, an abundant deposit of copper is obtained which is uniform and strongly adheres to the blank. Furthermore, the position and the limits of the metal deposit can be readily controlled by the position of the electrodes which are in contact with the disc and between which said deposit is formed. In particular, no deposit of copper has a tendency to form on the periphery of the disc, so that there is no danger of the copper-copper oxide rectifying contact being short circuited. It should be noted that the form of the alternating current used does not undergo any modification owing to the presence of the rectifying element since said current is entirely enclosed by the thin layer of reduced copper covering the disc and with which the two electrodes connected to the source of current are in contact, .so that the copper-copper oxide contact is not in the circuit.

I claim: I 1. A process of manufacturing dry rectiflers, in particular rectifying elements consisting of a copper part covered with a layer of copper oxide,

comprising oxidizing said parts 'in an oven at high temperature, at about 1000 C., dipping the oxidized parts into a bath brought to a temperature approximating that of the oxidation oven and which does not give oxygen off to the copper oxide at said temperature, but which is'at most slightly reducing, passing an electric current through the oxide layer and allowing said bath to gradually cool, the flow of electric current being maintained for the whole duration of crystallization of the oxide.

3. Aprocess according to claim 1, wherein the I5 intensity of the electric current used for the treatment is of the order of several amperes per cm. of the oxide surface treated.

4. A process according to claim 1 wherein the bath brought to oxidizing temperature comprises powdered graphite.

5. A process according to claim 1, wherein the bath brought to oxidizing temperature comprises a powdered copper oxide.

6. A process of manufacturing dry rectifiers, in particular rectifying elements consisting of a copper part covered with a layer of copper oxide, comprising oxidizing the copper part and then treating said part in a bath brought to the oxidizing temperature of copper and constituted of materials which do not give oxygen off to copper oxide at said temperature, but which are at most slightly reducing by passing an electric current through the oxide layer during the crystallization phase of the latter, and finally reduc-' ing by electro-chemical means the superficial layer of black cupric oxide formed during the oxidation phase by treating the part thus prepared in an electrolytic liquid and by placing the surface of the oxide to be reduced into contact with an anode made of a reducing metal.

7. A process of manufacturing dry rectifiers, in particular rectifying elements consisting of a copper part covered with a layer of copper oxide, comprising oxidizing the copper part and then treating said part in a bath brought to the oxidizing temperature of copper and constituted of materials which do not gi e oxygen of to copper oxide at said temperature, but which are at most slightly reducing by passing an electric current through the layer of oxide during the crystallization phase of the latter, and finally reducing by electro-chemical means the superficial layer of black cupric oxide formed during the oxidation phase by treating the part thus prepared in an electrolytic reducing liquid consisting of a conducting salt dissolved in water, and by placing the surface of the oxide to be reduced into contact with an iron anode.

8. A process of manufacturing dry rectifiers, in particular rectifying copper part covered with a layer of copper oxide, comprising oxidizing the copper part and then .treated part, and applying an elements consisting of a treating said part in a bath brought to the oxidizing temperature of copper and constituted of materials which do not give oxygen off to copper oxide at said temperature, but which are at most slightly reducing by passing an electric current through the layer of .oxide during the crystalization phase of the latter, reducing the superficial layer of black cupric oxide formed during the oxidation phase and covering by electro-plating the layer of reduced copper thus obtained, by treating the part in a solution of a salt of the metal to be deposited, by placing two electrodes made of different metals into contact with the layer of reduced copper, on the same face of the alternating diflerence of potential between said two electrodes.

9. A process of manufacturing dry rectifiers, in particular rectifying elements consisting of a copper part covered with a layer of copper oxide, comprising oxidizing the copper part and then treating said part in a bath brought to the oxidizing temperature of copper and constituted of materials which do not give oxygen 01! to copper oxide at said temperature, but which are at most slightly reducing by passing an electric current through the layer of oxide during the crystallization phase of the latter, reducing the superficial layer of black cupric oxide formed during. the oxidation phase and covering by electro-plating the layer of reduced copper thus obtained, by treating the part in a solution, which is made slightly reducing, of a salt of the metal to be tie-'- posited, by placing two electrodes respectively made of silver. and of zinc into contact with the layer of reduced copper, on the same face of the treated part, and by applying an alternating difference of potential between said two electrodes.

10. A process according to claim 1, wherein the bath brought to oxidizing temperature comprises metallic salts.

11. A process according to claim 1, wherein the bath brought to oxidizing temperature of copper' is rendered slightly reducing.

12. A process according to claim 1, wherein the bath brought to oxidizing temperature com- Q prises metal oxides.

, ETIENNE BARREZ. 

